1. Field of the Invention
The present invention relates to a hot nozzle for use with a mold for runnerless injection molding of plastic material, and more particularly to such hot nozzle structure in a hot runner system using a hot runner type runnerless mold.
2. Description of the Prior Art
Conventionally, many of plastic products used in daily necessaries, domestic electric apparatus, automobiles and other industry are manufactured by injection molding. In such injection molding, hot molten resin is injected under high pressure through runners into cavities of a mold, and after cooled, the molded product is removed from the mold. Inevitably, the molded product includes resin hardened in the runners, and therefore requires an additional operation to sever and remove the hardened resin. Such an operation is time- and labor-consuming work, and tends to reduce yields of product. Reclamation of the resin further requires cumbersome operations such as crushing. Particularly, all of these disadvantages are evident in multi-cavity molds.
Thus, it has been desired to provide a mold with no runner, or one having runners but eliminating the need for removing such runners at every molding operation. The hot runner system has been devised in order to meet this particular desire.
One known hot runner system is shown in FIG. 8 and as may be seen, a fixed mold half 101 and a movable mold half 102 define therebetween a plurality of cavities 103 (only one is shown in FIG. 8). The fixed mold half 101 has formed therein a longitudinal bore 104 in place of a runner which communicates with the cavity 103. A torpedo spreader 105 is mounted on the fixed mold half 101 at the inlet of the bore 104, with its elongated portion 105a protruding into the bore 104. The torpedo spreader 105 has formed therein passages 107 (only one is shown in FIG. 8) communicating with a molten resin dispensing manifold 106 through which molten resin is supplied from a nozzle of the injection molding machine (not shown). The molten resin passes through the passages 107 into a space 108 defined in the bore 104 between the fixed mold half 101 and the elongated portion 105a of the torpedo spreader 105, and flows into the cavity 103. The torpedo spreader 105 has formed therewithin an insertion hole 109 extending substantially up to the top end of the elongated portion 105 a for accommodating a heater 110 which can be energized by an external source of power through a lead wire 111.
In such a hot runner system employing a so-called internal heating system, the resin in the bore 104 corresponding to a runner is heated by the heater 111 through the elongated portion 105a of the spreader 105 to be always maintained in a molten state and hence, the molded article has no runner when removed from the cavity 103.
It should be noted, however, that in this hot runner system having an internal heating mechanism, the molten resin in the spacing 108 contiguous to the fixed mold half 101 tends to be hardened and stagnant as it is cooled by the fixed mold half 101. If the temperature of the heater is raised to prevent such stagnation, the resin in the vicinity of the elongated portion 105a of the spreader 105 will be thermally decomposed and clung to that portion. Thus, temperature control is difficult and power consumption is increased. Additionally, as resin is always stagnant in the bore 104, change of colors of resin will cause mixture of a color with the previous one, resulting in production of defective articles. Further, such a hot runner system has other disadvantages that the spreader body is difficult to machine, and that the construction is itself complicated.
The prior art hot runner system has also employed a socalled external heating mechanism in which a nozzle has therewithin a passage for molten resin, and a band heater is attached to the outside of the nozzle, or a heater is incorporated in the nozzle as a unit.
In this arrangement, however, heat of the heater tends to escape through the nozzle-mounting bore to the fixed mold half, resulting in increase in power consumption of the heater. In order to heat resin over the length of the nozzle-mounting bore, the heater must be provided over the length of the nozzle, and usually another heater is provided to heat the top end of the nozzle and facilitate the flow of resin into the cavity. Consequently, complexity of the structure and difficulty in temperature control are disadvantageously increased.